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Effect of Strain on Actomyosin Kinetics in Isometric Muscle Fibers

Division of Biomedical Sciences, Imperial College London, London SW7 2AZ, United Kingdom

Correspondence: Address reprint requests to M. A. Ferenczi, Imperial College London, Sir Alexander Fleming Bldg., South Kensington, London SW7 2AZ, UK. E-mail: m.ferenczi{at}imperial.ac.uk.

Investigations were conducted into the biochemical and mechanical states of cross-bridges during isometric muscle contraction. Rapid length steps (3 or 6 nm hs^–1) were applied to rabbit psoas fibers, permeabilized and isometric, at either 12°C or 20°C. Fibers were activated by photolysis of P³-1-(2-nitrophenyl)-ethyl ester of ATP infused into rigor fibers at saturating Ca²⁺. Sarcomere length, tension, and phosphate release were recorded—the latter using the MDCC-PBP fluorescent probe. A reduction in strain, induced by a rapid release step, produced a short-lived acceleration of phosphate release. Rates of the phosphate transient and that of phases 3 and 4 of tension recovery were unaffected by step size but were elevated at higher temperatures. In contrast the amplitude of the phosphate transient was smaller at 20°C than 12°C. The presence of 0.5 or 1.0 mM added ADP during a release step reduced both the rate of tension recovery and the poststep isometric tension. A kinetic scheme is presented to simulate the observed data and to precisely determine the rate constants for the elementary steps of the ATPase cycle.

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